Japanese encephalitis (JE) virus and dengue virus are closely related flaviviruses but interaction between them is scarcely studied in humans. The objective of this study was to compare the outcome of JE patients who are positive and negative for dengue IgG antibodies. Patients of acute encephalitis syndrome (AES) fulfilling predecided inclusion and exclusion criteria underwent a detailed standardized workup incorporating JE IgM testing in cerebrospinal fluid (CSF) or serum. Dengue IgG in serum was tested in all. Outcomes and clinical features were compared between JE patients who were dengue IgG positive and negative in hospital and after 3 months. A total of 182 patients of JE were enrolled over 3 seasons. After excluding equivocal cases, hospital outcome was compared between 105 dengue IgG-positive and 50 dengue IgG-negative patients. Dengue IgG-positive patients had a significantly better outcome (relative risk 1.4; 95% confidence interval 1.1–1.8). On follow-up after 3 months, again, outcome was significantly better among 95 dengue IgG-positive patient than 47 dengue IgG-negative patients (relative risk 1.3; 95% confidence interval 1.01–1.8). Presence of pre-existing dengue IgG antibodies in JE patients is associated with improved outcomes.